Heretofore a laser altimeter system has been proposed for determining the altitude of a flight simulation probe over a model board. Such a prior art system is disclosed in an article entitled, "Probe Protection In Camera/Model Visual Systems" appearing in the Proceedings of the 1980 Summer Computer Simulation Conference, Olympic Hotel, Seattle, Wash., Aug. 25-27, 1980.
In this system, a laser beam is directed vertically from the flight simulator TV camera probe along a side of the probe to strike a point on the model board radially displaced from a point directly below the center line of the probe, hereafter referred to as a plumb line. The incident beam produces a beam spot on the terrain of the model which is thence imaged onto a linear array sensor. As the height of the probe is varied, while holding the probe otherwise stationary, from a point of minimum altitude to a point of maximum altitude the beam spot traverses a vertical imaginary line focused onto the linear array sensor. The position of the imaged beam spot along the linear array sensor is representative of altitude. A major problem with this system is that the altitude being measured is not the altitude of the probe (altitude measured along the plumb line) but rather the altitude of the laser beam source which is displaced horizontally from the probe. The actual distance between the plumb line and the laser beam must be multiplied by the scale of the model. Error will be produced whenever the terrain elevation at the plumb line differs from the terrain elevation at the laser beam. For example, an appreciable error would exist if the plumb line was over a depression and the laser beam impinged on a hill, mountain or tall building.
Thus, it is desirable to obtain a probe height sensor which more accurately measures the height of the probe above the terrain directly below the probe.